Optical modulation on a silicon waveguide platform is well known including waveguide modulators based upon: electro-optic, quantum confined Stark, Franz-Keldysh, and carrier plasma dispersion.
Waveguide modulators can include silicon, III-V, other semiconductors, and polymer waveguides. They are typically symmetrical structures, although doped areas may not be symmetrical (e.g. U.S. Pat. No. 7,085,443, PN phase shift modulator) where the doping of silicon is asymmetric across the waveguide).
For modulators with a silicon/germanium (SiGe) medium, it is known that the Franz-Keldysh (FK) effect with lumped electrodes is used to achieve small size, high speed, low driving power consumption and ease of manufacture at 1550 nm. A silicon/germanium region provides an electro-absorption effect that is advantageous when operation is required in the C- and L-bands of the optical spectrum.
Several possible modulator waveguide structures have been proposed including U.S. Pat. No. 8,346,028 where the active waveguide is made of (typically) SiGe and the SiGe waveguides are doped on their sides to provide effective electric contacts and to generate the necessary electric field in the SiGe waveguide.
However, such a structure suffers from the drawback that the optical mode is forced to be in close proximity to doping on both sides of the waveguide, which causes optical losses.